A seal system particularly suitable for a process valve, in which the intermediate chamber formed between two sliding seal gaskets arranged in series is connected to a static separator of elastic plenum chamber or bag type filled with a biodegradable, non-contaminating, non-aggressive auxiliary lubricant fluid, the internal plenum chamber or bag, which is of very high elasticity, being connected to the process fluid and cooperating with a movable head which itself cooperates with a position sensor. A modified embodiment is also described.